1. Field of the Invention
The present invention is related to a bridgeless active power factor corrector, and more particularly to a bridgeless active power factor corrector with a logic control.
2. Description of the Prior Art
Causing the AC input current to be in phase with the AC input voltage, a resistor consumes real power. On the contrary, an inductor or a capacitor stores imaginary power because of causing the AC input current to be in quadrature with the AC input voltage. More simply, only real power does a resistive load consume. Not only does a non-resistive load consume real power but it also stores imaginary power. The imaginary power leads to an added AC input current flowing through power lines and an increased line loss shouldered by power companies. Therefore, it is strongly requested by power companies the power factor (PF) of large electrical equipments be strictly corrected to an acceptable value. Generally speaking, to correct the power factor is to align the AC input current to be in phase with the AC input voltage. In switching power supplies drawing more than 75 W from power grids, an Active Power Factor Corrector (APFC) is nearly an essential device reforming the AC input current to be both in phase and in shape with the AC input voltage so as to meet a stringent requirement for a power factor higher than 0.95.
It is shown in FIG. 1a prior bridge-based APFC comprising a bridge rectifier 10 and a conventional APFC 11. A bridge rectifier 10, an AC/DC Conversion (ADC) device, rectifies an AC sinusoidal input voltage across a first Vi1 and a second input voltage terminal Vi2 into a DC sinusoidal output voltage on an input filtering capacitor C11. A conventional APFC 11 reforms the AC input current to be both in phase and in shape with the AC input voltage as well as boosts a lower DC sinusoidal input voltage on an input filtering capacitor C11 to a higher DC constant output voltage on an output filtering capacitor C12.
To conveniently explain how electric energy is stored to and released from a boost inductor, it is assumed throughout this text the horizontal and the vertical axis in a Cartesian coordinate system (I-V coordinate system) respectively stand for the inductor current and voltage on the boost inductor. The boost inductor L11 operates in either the first or the fourth quadrant because the inductor current is always positive while the inductor voltage is either positive to store or negative to release energy. When the high frequency switch controller 12 switches on the boost transistor Q11, the inductor current, flowing through the input filtering capacitor C11, the boost inductor L11, and the boost transistor Q11, stores energy to the boost inductor L11, operating in the first quadrant. When the high frequency switch controller 12 switches off the boost transistor Q11, the inductor current, flowing through the input filtering capacitor C11, the boost inductor L11, the boost diode D11, and the output filtering capacitor C12, releases energy from the boost inductor L11, operating in the fourth quadrant. Not only do the high frequency switch controller 12 and the conventional APFC 11 reform the AC input current to correct the power factor but they also regulate the DC output voltage to supply a DC/DC converter so that a complex system looks like a simple resistor in the eyes of power grids.
Please take a closer look at the bridge rectifier 10. In order to set up a polarity reference for the following description to cite, a positive/negative half period is defined as the potential of Vi1 is higher/lower than that of Vi2. During the positive/negative half period, the upper left/right and the lower right/left diode rectifiers conduct the AC input current. Therefore, the prior bridge-based APFC, barely acceptable in a low to medium power range, suffers from a diode rectifier conduction loss seriously hindering power engineers from designing high-power, high-efficiency power supplies. As the power level grows with days, the bridge rectifier also begins becoming a hot potato very difficult to deal with. A prior bridgeless APFC needing no bridge rectifier for ADC breaks through such a bottleneck.
It is shown in FIG. 2 a prior bridgeless APFC implicitly making use of the intrinsic body diode internally oriented from the source to the drain terminal of a boost transistor as well as concurrently implementing both ADC and APFC without the need for a bridge rectifier. The core(s) and winding(s) of a boost inductor L21 can be lumped together either on one path between the first input voltage terminal Vi1 and a first connecting node V1 or on another path between the second input voltage terminal Vi2 and a second connecting node V2 or they can be distributed apart over both paths. An output filtering capacitor C21 is connected between the output Vo and the reference voltage terminal Vref. Boost diodes D21 and D22 implemented with Silicon Carbide Schottky Diodes (SCSD) as well as boost transistors Q21 and Q22 implemented with N-channel Metal Oxide Semiconductor Field Effect Transistors (NMOSFET) are connected in a bridge configuration placed between the boost inductor L21 and the output filtering capacitor C21. A high frequency switch controller 22 simultaneously switches on/off the boost transistors Q21 and Q22.
During the positive/negative half period, the boost inductor L21 operates in either the first/third or the fourth/second quadrant because the inductor current is always positive/negative while the inductor voltage is either positive/negative to store or negative/positive to release energy. When the high frequency switch controller 22 simultaneously switches on the boost transistors Q21 and Q22, the inductor current, flowing through the input voltage terminal Vi1/Vi2, the boost inductor L21, the channels of the boost transistors Q21, Q22, and the input voltage terminal Vi2/Vi1, stores energy to the boost inductor L21, operating in the first/third quadrant. When the high frequency switch controller 22 simultaneously switches off the boost transistors Q21 and Q22, the inductor current, flowing through the input voltage terminal Vi1/Vi2, the boost inductor L21, the boost diode D21/D22, the output filtering capacitor C21, the body diode of the boost transistor Q22/Q21, and the input voltage terminal Vi2/Vi1, releases energy from the boost inductor L21, operating in the fourth/second quadrant.
In the prior bridge-based APFC, through two rectifier diodes flows the AC input current rectified by a bridge rectifier whether storing/releasing energy to/from the boost inductor. In the prior bridgeless APFC, through one boost and one body diode flows the AC input current not rectified by a bridge rectifier when releasing energy from the boost inductor. Needing no bridge rectifier for ADC, the prior bridgeless APFC has a higher efficiency than the prior bridge-based APFC does. However, it is still a fly in the ointment the inductor/AC input current in the prior bridgeless APFC releases energy via the body diodes, causing a body diode conduction loss. Using the channels of the boost transistors to release energy, the present invention proposes a new topology of bridgeless APFC to reduce the body diode conduction loss.